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      7.5.&nbsp;Creating Custom Symlinks to Devices
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        Linux From Scratch - Version 7.3
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      <h3>
        Chapter&nbsp;7.&nbsp;Setting Up System Bootscripts
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    <div class="sect1" lang="en" xml:lang="en">
      <h1 class="sect1">
        <a id="ch-scripts-symlinks" name="ch-scripts-symlinks"></a>7.5.
        Creating Custom Symlinks to Devices
      </h1>
      <div class="sect2" lang="en" xml:lang="en">
        <h2 class="sect2">
          7.5.1. CD-ROM symlinks
        </h2>
        <p>
          Some software that you may want to install later (e.g., various
          media players) expect the <code class="filename">/dev/cdrom</code>
          and <code class="filename">/dev/dvd</code> symlinks to exist, and
          to point to a CD-ROM or DVD-ROM device. Also, it may be convenient
          to put references to those symlinks into <code class=
          "filename">/etc/fstab</code>. Udev comes with a script that will
          generate rules files to create these symlinks for you, depending on
          the capabilities of each device, but you need to decide which of
          two modes of operation you wish to have the script use.
        </p>
        <p>
          First, the script can operate in &ldquo;<span class=
          "quote">by-path</span>&rdquo; mode (used by default for USB and
          FireWire devices), where the rules it creates depend on the
          physical path to the CD or DVD device. Second, it can operate in
          &ldquo;<span class="quote">by-id</span>&rdquo; mode (default for
          IDE and SCSI devices), where the rules it creates depend on
          identification strings stored in the CD or DVD device itself. The
          path is determined by Udev's <span class=
          "command"><strong>path_id</strong></span> script, and the
          identification strings are read from the hardware by its
          <span class="command"><strong>ata_id</strong></span> or
          <span class="command"><strong>scsi_id</strong></span> programs,
          depending on which type of device you have.
        </p>
        <p>
          There are advantages to each approach; the correct approach to use
          will depend on what kinds of device changes may happen. If you
          expect the physical path to the device (that is, the ports and/or
          slots that it plugs into) to change, for example because you plan
          on moving the drive to a different IDE port or a different USB
          connector, then you should use the &ldquo;<span class=
          "quote">by-id</span>&rdquo; mode. On the other hand, if you expect
          the device's identification to change, for example because it may
          die, and you would replace it with a different device with the same
          capabilities and which is plugged into the same connectors, then
          you should use the &ldquo;<span class="quote">by-path</span>&rdquo;
          mode.
        </p>
        <p>
          If either type of change is possible with your drive, then choose a
          mode based on the type of change you expect to happen more often.
        </p>
        <div class="admon important">
          <img alt="[Important]" src="../images/important.png" />
          <h3>
            Important
          </h3>
          <p>
            External devices (for example, a USB-connected CD drive) should
            not use by-path persistence, because each time the device is
            plugged into a new external port, its physical path will change.
            All externally-connected devices will have this problem if you
            write Udev rules to recognize them by their physical path; the
            problem is not limited to CD and DVD drives.
          </p>
        </div>
        <p>
          If you wish to see the values that the Udev scripts will use, then
          for the appropriate CD-ROM device, find the corresponding directory
          under <code class="filename">/sys</code> (e.g., this can be
          <code class="filename">/sys/block/hdd</code>) and run a command
          similar to the following:
        </p>
        <pre class="userinput">
<kbd class="command">udevadm test /sys/block/hdd</kbd>
</pre>
        <p>
          Look at the lines containing the output of various *_id programs.
          The &ldquo;<span class="quote">by-id</span>&rdquo; mode will use
          the ID_SERIAL value if it exists and is not empty, otherwise it
          will use a combination of ID_MODEL and ID_REVISION. The
          &ldquo;<span class="quote">by-path</span>&rdquo; mode will use the
          ID_PATH value.
        </p>
        <p>
          If the default mode is not suitable for your situation, then the
          following modification can be made to the <code class=
          "filename">/etc/udev/rules.d/83-cdrom-symlinks.rules</code> file,
          as follows (where <em class="replaceable"><code>mode</code></em> is
          one of &ldquo;<span class="quote">by-id</span>&rdquo; or
          &ldquo;<span class="quote">by-path</span>&rdquo;):
        </p>
        <pre class="userinput">
<kbd class="command">sed -i -e 's/"write_cd_rules"/"write_cd_rules <em class=
"replaceable"><code>mode</code></em>"/' \
    /etc/udev/rules.d/83-cdrom-symlinks.rules</kbd>
</pre>
        <p>
          Note that it is not necessary to create the rules files or symlinks
          at this time, because you have bind-mounted the host's <code class=
          "filename">/dev</code> directory into the LFS system, and we assume
          the symlinks exist on the host. The rules and symlinks will be
          created the first time you boot your LFS system.
        </p>
        <p>
          However, if you have multiple CD-ROM devices, then the symlinks
          generated at that time may point to different devices than they
          point to on your host, because devices are not discovered in a
          predictable order. The assignments created when you first boot the
          LFS system will be stable, so this is only an issue if you need the
          symlinks on both systems to point to the same device. If you need
          that, then inspect (and possibly edit) the generated <code class=
          "filename">/etc/udev/rules.d/70-persistent-cd.rules</code> file
          after booting, to make sure the assigned symlinks match what you
          need.
        </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <h2 class="sect2">
          7.5.2. Dealing with duplicate devices
        </h2>
        <p>
          As explained in <a class="xref" href="udev.html" title=
          "7.4.&nbsp;Device and Module Handling on an LFS System">Section&nbsp;7.4,
          &ldquo;Device and Module Handling on an LFS System&rdquo;</a>, the
          order in which devices with the same function appear in
          <code class="filename">/dev</code> is essentially random. E.g., if
          you have a USB web camera and a TV tuner, sometimes <code class=
          "filename">/dev/video0</code> refers to the camera and <code class=
          "filename">/dev/video1</code> refers to the tuner, and sometimes
          after a reboot the order changes to the opposite one. For all
          classes of hardware except sound cards and network cards, this is
          fixable by creating udev rules for custom persistent symlinks. The
          case of network cards is covered separately in <a class="xref"
          href="network.html" title=
          "7.2.&nbsp;General Network Configuration">Section&nbsp;7.2,
          &ldquo;General Network Configuration&rdquo;</a>, and sound card
          configuration can be found in <a class="ulink" href=
          "http://www.linuxfromscratch.org/blfs/view/svn/postlfs/devices.html">
          BLFS</a>.
        </p>
        <p>
          For each of your devices that is likely to have this problem (even
          if the problem doesn't exist in your current Linux distribution),
          find the corresponding directory under <code class=
          "filename">/sys/class</code> or <code class=
          "filename">/sys/block</code>. For video devices, this may be
          <code class="filename">/sys/class/video4linux/video<em class=
          "replaceable"><code>X</code></em></code>. Figure out the attributes
          that identify the device uniquely (usually, vendor and product IDs
          and/or serial numbers work):
        </p>
        <pre class="userinput">
<kbd class="command">udevadm info -a -p /sys/class/video4linux/video0</kbd>
</pre>
        <p>
          Then write rules that create the symlinks, e.g.:
        </p>
        <pre class="userinput">
<kbd class=
"command">cat &gt; /etc/udev/rules.d/83-duplicate_devs.rules &lt;&lt; "EOF"
<code class="literal">
# Persistent symlinks for webcam and tuner
KERNEL=="video*", ATTRS{idProduct}=="1910", ATTRS{idVendor}=="0d81", \
    SYMLINK+="webcam"
KERNEL=="video*", ATTRS{device}=="0x036f", ATTRS{vendor}=="0x109e", \
    SYMLINK+="tvtuner"
</code>
EOF</kbd>
</pre>
        <p>
          The result is that <code class="filename">/dev/video0</code> and
          <code class="filename">/dev/video1</code> devices still refer
          randomly to the tuner and the web camera (and thus should never be
          used directly), but there are symlinks <code class=
          "filename">/dev/tvtuner</code> and <code class=
          "filename">/dev/webcam</code> that always point to the correct
          device.
        </p>
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